Enhancement of electric fields around metal particles is a topic of current scientific and technological interest. For example, surface enhanced Raman spectroscopy (“SERS”) is a well-known spectroscopy technique that utilizes an enhanced electric field near a specially prepared, roughened metal surface or metal particles to increase the amount of inelastically scattered Raman radiation from an analyte. In SERS, the analyte is adsorbed onto or placed adjacent to an activated metal surface or structure. Irradiation of the analyte and the metal surface or particles with electromagnetic radiation (“EMR”) of a selected frequency excites surface plasmon polaritons (“SPPs”) in the metal surface or particles.
During SERS, the analyte experiences the intense, localized electric field of the SPP, and Raman photons characteristic of the analyte are inelastically scattered from the analyte. The enhanced electric field is considered one significant factor for the relatively increased Raman radiation compared to when Raman spectroscopy is practiced without the metal surface or particles. For example, the enhanced electric field from the metal surface may enhance the Raman scattering intensity by factors of between 103 and 106.
Recently, Raman spectroscopy has been performed employing randomly oriented metal nanoparticles, such as nanometer scale needles, islands, and wires, as opposed to a simple roughened metal surface, for enhancing electric fields. The intensity of the Raman scattered photons from a molecule adsorbed on such a metal surface may be increased by a factor greater than 106. At this level of sensitivity, Raman spectroscopy can be used to detect minute amounts of species and is referred to as nano-enhanced Raman spectroscopy (“NERS”).
As can be appreciated from the discussion above about SERS and NERS, enhancement of electric fields around metal particles can be of significant utility. In addition to SERS and NERS, enhancement of electric fields can be used in other applications, such as infrared spectroscopy, sensors, Raman imaging systems, nanoantennas, and many other applications. Accordingly, researchers and developers of electric-field-enhancement structures can appreciate a need for improved electric-field-enhancement structures that may be used in a wide variety of applications, such as sensors, Raman spectroscopy systems, and many other applications.